Antielastin antibody was used, in conjunction with the immonoperoxidase and the immunoferritin methods, to study the localization of elastin in untreated and in elastase-treated elastic fibers of nuchal ligament and aorta of fetal and young adult sheep. The results obtained show: 1) that the immunohistochemical method is sufficiently sensitive to detect amounts of elastin that cannot be identified by ordinary electron microscopic staining; 2) that partially degraded elastic fibers stain more intensely and homogeneously than do normal elastic fibers because their elastin is more loosely arranged and permits the antibody to penetrate, and 3) that the surfaces of microfibrils associated with elastic fibers also contain small amounts of elastin.